Low power heater



March 23, 1965 R. J. NEY

LOW POWER HEATER Filed May 28, 1962 FIG.3.

FIG.5.

IN VENTOR ROBERT J I00 I50 POWER INPUT (MILLIWATTS) NEY,

BY ms TORNEY.

l 2 HEATER VOLTS mm! qbkuze United States Patent Ofiice 3,l75,l l8 Patented Mar. 23, 196% 3,175,1ll LBW ltlWETl HEATER Robert .l. Ney, North Syracuse, Nilfi, assignor to General Electric Company, a corporation of New York Filed May 28, 1962, Ser. N 197,981 5 Claims. (Ql. 313-349) The present invention relates to cathode ray tubes and more particularly to an improved low power emitter or heater-cathode assembly particularly suitable for cathode ray tubes.

A low input power heater cathode assembly for use in cathode ray tubes such as television picture tubes and camera tubes has long been recognized as desirable. In television picture tubes operating in transistorized batterypowered television receivers, low heater power is particularly desirable in order to minimize battery power drain. In the camera tube field infrared and low light-level sensitive devices may be limited in performance, particularly in signal-to-noise ratio, by the extraneous heat undesirably radiated from the electron beam generating portion of the tube. Moreover in camera and other tubes required to operate with an electron beam-scanned target or other elements which are artificiall cooled, the problem of suppression of undesired heat radiation from the electron gun is substantially decreased by reducing the input heater power requirements.

In my copending commonly-assigned application Serial No. 159,718, filed December 15, 1961, now US. Patent 3,155,865, there is described and claimed a low input power heater-cathode assembly particularly suitable for operation at input power levels of the order of 200 milliwatts and having a number of advantages deriving from it mechanical ruggedness and relative ease of manufacture and assembly. The present invention embodies certain improvements over the apparatus described and claimed in the aforesaid copending application, and provides a heater-cathode assembly particularly suitable for stable and reliable operation at emission current levels of the order of 1,000 microamperes and at extremely low input power levels of the order of 50-100 milliwatts.

Accordingly, a principal object of the present invention is to provide an improved heater-cathode assembly for cathode ray tubes and the like having minimal heater input power requirements.

Another object is to provide a heater-cathode assembly of the character described which is mechanically rugged, relatively low cost, and relatively easy to manufacture and assemble.

Another object is to provide a heater cathode assembly of the character described in which the heat developed by the heater is efficiently transferred to the thermally emissive cathode with a minimum of extraneous heat loss from the heater l ads or otherwise.

Another object is to provide a heater-cathode assembly of the foregoing character which has a high resistance to surge current when initially energized, and which has a good capacity to resist burnout under conditions of overvoltage.

These and other objects of the present invention will be apparent from the following description and the accompanying drawing wherein:

FEGURE 1 is a sectional view of a portion of an electron gun including a heater-cathode assembly constructed in accordance with the present invention;

FIGURE 2 is an enlarged fragmentary sectional view of a portion of the structure of FIGURE 1;

FIGURE 3 is an enlarged cutaway perspective view of a portion of the structure of FIGURE 1;

FIGURE 4 is a view of the structure of FIGURE 2 taken on the line i4 th reof;

FIGURE 5 is a graph illustrating certain relationships of emission current, heater power, and heater voltage in a cathode-heater assembly according to my invention; and

FIGURE 6 is a fragmentary view to a diminished scale of an alternative form of a portion of the structure of FlGURE 4.

Referring to FIGS. 1 through 4 of the drawings, the low heater power heater-cathode assembly of the present invention includes an inverted cup-shaped cathode 2 on the exterior surface of the transverse top wall 4 of which is provided a suitable thermally electron-emissive coating 6 such as a layer of one or more alkaline earth metal carbonates reducible during conventional cathode activation heat treatment to the alkaline earth metal or metal oxide state. Depending from the top wall 4 is an integral cylindrical skirt 8 forming the side wall of the cathode cup and shown as having a length about equal to onethird the top wall diameter. The side wall may be substantially thinner than the top wall, as shown, so as to minimize heat transfer from the top to the bottom of the side wall. For example in an assembly having a top wall diameter of 50 mils the top wall may have a thickness of 2 mils and the side wall a thickness of approximately 0.1 mil. Alternatively both the top wall 4 and side wall 3 may be as thin as practicable, for example 0.1 to 0.5 mil.

The cathode is supported by a plurality of depending legs ltl shown for example as three in number and which may consist of small cross-section metallic electrically conductive material of low heat conductivity, such as Nichrome. The three legs are secured as by welds to the side wall 3 of the cathode cup, and are arranged pyramidally with approximately isosceles tetrahedral angles to ailord optimum strength and thereby permit the legs ill to be of minimum cross section.

Preferably the legs 10 should have as long a length as possible, in proportion to the cross sectional area of each, so as to reduce the conduction heat loss from the cathode. Desirably the length-to-area ratio of the legs may be over 30,000 units per square unit for each leg and in practice it has been found that the conduction heat loss through the legs is thereby reduced to less than 2% of the total input to the cathode. The legs 10 are attached at their lower ends, as by means of welds, to an annular support le' of electrically conductive material from which in turn a single cathode external lead 18 may extend through a supporting electrically insulative wafer 20.

Situated within the inverted cup-shaped cathode is the heater 3%. The heater 3%) includes a filamentary heating element 34 from which extend heater leads 32. The heating element 3 2- and heater leads 3?. are formed of a common integral continuous elongate electrical conductor, which may be for example a. single-filament tungsten-alloy wire. In order to minimize heat loss from the heater leads 32 relative to desired heat transfer to the cathode, the heater 3t is arranged with its middle heating element portion 34 wound in a coil consisting of a relatively large number of turns and having such dimensions as to fit entirely within the inverted cup-shaped cathode 2 with the axis of the coil 34 generally parallel to the transverse top wall 4. i

To enhance its ruggedness and facilitate manufacture, the heating element coil 34 is wound on an electrically insulative support 366 preferably consisting of a rectangular slab of a suitable ceramic such as alumina, so dimensioned as to fit within the cathode cup 2. In an actual embodiment employing a 50 mil outside diameter cathode cup, for example, and a heater coil support slab as having dimensions of about 30 by 30 mils square and 5 to 10 mils thick, I have found it possible to wind some 40 turns of one-half mil diameter tungsten alloy wire on the support as, thereby providing a total length of wire on the support 36 of some three inches and fitting entirely within the cathode cup. Preferably the slab 36 is so dimensioned and arranged that the lower courses of the turns of coil 34 are spaced from the upper courses of the coil turns no more than the upper courses are spaced from the underside of top wall 4, so as to maximize heat transfer to the top wall from both upper and lower courses' The edges of the core slab are preferably somewhat rounded oil? so as to minimize the sharpness of bends and thereby reduce resulting stresses in the turns of the wire forming the heater coil To avoid fracture of the wire during temperature cycling of the assembly due to differences in thermal expansion of the wire and core 36, the wire must not be wrapped too tightly around the core 36.

Further according to the invention, the heater coil 34 and the core slab 36 on which it is wound are encapsulated or potted in an electrically insulative material 46 of good heat conductivity, such as a slurry of alumina in a suitable binder such as nitrocellulose or isobutyl methacrylate and an appropriate solvent, the binder subsequently being removed during drying and baking. Additional potting material 42 of good heat conductivity is provided between the heater coil capsule and the underside of the cathode cup top wall 4 so as to insure intimate Contact and maximize heat transfer between the heater and the top wall The potting material 42 may also be, for example, a slurry of alumina and a suitable vehicle which may be placed in position in liquid form and subsequently dried to hardness by baking. The potting material 469 is so dimensioned and arranged, however, and the quantity of potting material 42 is kept suliiciently small, as to avoid contact with all or as much as possible of the side wall 8 of the cathode cup so as to thereby minimize heat transfer by conduction from the encapsulated heater coil directly to side wall 8. This spacing of the side wall 8 from the heater coil capsule substantially limits heat transfer from the heater coil capsule to the side wall to a transfer solely by radiation, rather than conduction. As a result the internal surface of the side wall can reflect or reradiate the heat energy back to the heater coil capsule, and hence very little net heat loss from the heater coil capsule directly to the side walls 8 of the cathode cup 2 is experienced. To further enhance the reradiation or reflection capability of the side walls 6, the interior surface thereof may be polished in any suitable manner.

Extraneous heat loss from the cathode cup 2 is further minimized by extending the electron emissive coating 6 of the cathode down the side walls 8 of the cathode cup as a heat retaining blanket. This covering of the exterior surface of the side walls 8 by the cathode coating material serves as a convenient low cost way to inhibit and minimize radiation heat loss from the side walls 8 because at temperatures within the temperature range of normal operation of the cathode assembly, for example 700 to 1200 C., desirable cathode coating materials 6 such as barium or other alkali metal oxides have a lower heat emissivity coefficient than desirable cathode substrate metals such as nickel.

The thickness of the potting material between the heater coil and the underside of the cathode top wall is preferably a minimum consistent with securing good intimate heat conducting contact to the underside of the heater input terminals consisting of a pair of relatively thick electrically conductive 52' mounted in the support wafer Each heater lead 32 is connected as by means of a weld to the top of a respective post 52. To facilitate welding of the fine wire leads 32, each lead is sandwiched between a metal foil patch 54 and the respective posts 52, and the patch 54, lead 32 and post 52 are then welded together. To minimize excessive bending and resulting stresses in the heater leads during temperature cycling of the heater-cathode assembly a bight portion 56 of relatively large radius of curvature is provided in each heater lead between its point of emergence from the heater capsule and point of connection to the respective heater input terminal post 52.

FIGURE 5 shows a graph of electron emissive current from the cathode plotted against heater voltage and heater power input. As evident from "FIGURE 5 a substantial variation in heater power and heater input voltage is permissible without incurring more than a relatively modest variation in emission current from the cathode. Thus a heater according to the invention can tolerate a wide variation in battery voltage, and has a high capacity for withstanding overvoltage conditions without burning out. Also the relatively long length of filament in the coil 34 reduces the current requirement for a given power input, hence also reduces surge currents which might otherwise shorten the life of the heater during warrnup.

FIGURE 6 shows an alternative construction in which the coiled portion 34a of the heating element is confined to the mid-portion of the core 36a. With such an arrangement the extraneous heat losses are further reduced because the peripheral region of the top wall of the cathode and the side Walls 8 are cooler, and less heat is lost by radiation from these areas. This has the additional advantage of reducing the effective emitting area of the top wall to less than the entire area of the top wall where a minimum size emitting area is desired, while avoiding difficulties attendant upon reducing the entire overall diameter of the structure in order to secure such reduced emitting area. The structure shown in FIGURE 6 has the further advantage that with the reduced size of the emitting area the radiation heat losses from the top wall are likewise reduced.

It will also be appreciated that by reducing the thickness of the core 36 the underside of each turn of coil 34 can be brought closer to the emitting surface of the cathode, for a corresponding reduction in heat losses and increase in efficiency of heat transfer to the emitting surface. Correspondingly, with a reduction in the thickness of core 36, the radiation-shielding side walls 8 can be shortened, with a beneficial reduction in radiation area, and hence radiation heat losses, from the side walls.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than the illustrative embodiments heretofore described. Accordingly it is to be understood that the scope of the invention is not limited by the detatils of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

=1. A heater-cathode assembly for an electron gun or the like comprising an inverted cup-shaped cathode having a side wall, a transverse top wall surmounting said side wall for supporting an emission surface of thermionic electron emissive material, a continuous elongate conductive heating element arranged in a coil, said coil being embedded within an encapsulation of electrically insulative thermally conductive material, said encapsulation being disposed Within said cathode with the axis of said coil extending generally parallel to said top wall, the lower courses of the turns of said coil being downwardly spaced from the upper courses of the turns of said coil no more than the spacing of said upper courses from the underside of said top wall, the sides of said encapsulation being inwardly spaced from the side walls of said cathode cup to minimize heat transfer from said encapsulation to the side walls of said cathode, the top of said capsule being in intimate heat transferring contact with the underside of the top wall of said cathode, and conductors extending from said coil externally of said encapsulation and forming leads for energizing said heating element.

2. A heater-cathode assembly for an electron gun or the like comprising an inverted cup-shaped cathode having a side wall, a transverse top wall surmounting said side wall for supporting an emission surface of thermionic electron emissive material, a continuous elongate conductive heating element including end portions separated by a middle portion, said middle portion being arranged in a coil, an encapsulation of electrically insulative thermally conductive material surrounding said coil and supporting said coil within said cathode with the axis of said coil extending generally transversely, said encapsulation being spaced from the inner surfaces of the side walls of said cathode cup and being in intimate heattransferring contact with the underside of the top wall of said cathode cup, said heating element end portions extending from said coil externally of said encapsulation and forming leads for energizing said heating element, and a layer of said electron emissive material covering the outside surface of said side wall.

3. A heater-cathode assembly for an electron gun or the like comprising an inverted cup-shaped cathode having a side wall, a transverse top wall surmounting said side wall and adapted to support a layer of thermionic electron emissive material, an electrically insulative support, a continuous integral elongate electrically conductive heating element including end portions separated by a middle portion, said middle portion being coiled around said support with the axis of said coil extending generally transversely, said end portions depending from said middle portion to form integral leads for energizing said heating element, and an encapsulation of thermally conductive electrically insulative material covering said coil and support, said encapsulation being disposed within said cupshaped cathode with the side walls of encapsulation inwardly spaced from the side wall of said cathode and the top surface of said encapsulation in intimate heat transferring contact with the underside of said top wall of said cathode, and a plurality of legs connected to said cathode for supporting the same.

4. A heater-cathode assembly for an electron gun or the like comprising a downwardly concave cathode having a transverse top wall for supporting an emission surface of thermionic electron emissive material and a side wall depending from said top wall, a continuous elongate conductive heating element arranged in a coil, said coil being covered by an encapsulation of electrically insulative thermally conductive material, said encapsulation being disposed within said cathode with the axis of said coil extending generally parallel to said top wall, the sides of said encapsulation being inwardly spaced from the side walls of said cathode to minimize heat transfer from said encapsulation to the side walls of said cathode, the top of said capsule being in intimate heattransferring contact with the underside of the top wall of said cathode, and conductors extending from said coil externally of said encapsulation and forming leads for energizing said heating element.

5. A heater-cathode assembly for an electron gun or the like comprising an inverted cup-shaped cathode having a side wall, a transverse top wall surmounting said side Wall and adapted to support a layer of thermionic electron emissive material, an electrically insulative support, a continuous integral elongate electrically conductive heating element including end portions separated by a middle portion, said middle portion being coiled around only the central portion of said support with the axis of said coil extending generally transversely, said end portions depending from said middle portion to form integral leads for energizing said heating element, and an encapsulation of thermally conductive electrically insulative material covering said coil and support, said encapsulation being disposed within said cupshaped cathode with the side walls of said encapsulation spaced from the side wall of said cathode and the top surface of said encapsulation in intimate heat transferring contact with the underside of said top Wall of said cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,068,287 1/37 Gabor 313-340 X 2,075,910 4/37 Robinson.

2,510,639 6/ Koch 313-340 X 2,542,657 2/51 Gall 313-340 2,782,334 2/57 Gardner 313-346 X 2,899,591 8/59 Stein 313-340 X 2,953,701 9/60 Gale 313-346 3,056,900 10/ 62 Tyson 313-346 DAVID J. GALVIN, Primary Examiner.

JAMES D. KALLAM, Examiner. 

1. A HEATER-CATHODE ASSEMBLY FOR AN ELECTRON GUN OR THE LIKE COMPRISING AN INVERTED CUP-SHAPED CATHODE HAVING A SIDE WALL, A TRANSVERSE TOP WALL SURMOUNDING SAID SIDE WALL FOR SUPPORTING AN EMISSION SURFACE OF THERMIONIC ELECTRON EMISSIVE MATERIAL, A CONTINUOUS ELONGATED CONDUCTIVE HEATING ELEMENT ARRANGED IN A COIL, SAID COIL BEING EMBEDDED WITHIN AN ENCAPSULATIION OF ELECTRICALLY INSULATIVE THERMALLY CONDUCTIVE MATERIAL, SAID ENCAPSULATION BEING DISPOSED WITHIN SAID CATHODE WITH THE AXIS OF SAID COIL EXTENDING GENERALLY PARALLEL TO SAID TOP WALL, THE LOWER COURSES OF THE TURNS OF SAID COIL BEING DOWNWARDLY SPACED FROM THE UPPER COURSES OF THE TURNS OF SAID COIL NO MORE THAN THE SPACING OF SAID UPPER COURSES FROM THE UNDERSIDE OF SAID TOP WALL, THE SIDES OF SAID ENCAPSULATING BEING INWARDLY SPACED FROM THE SIDE WALLS OF SAID CATHODE CUP TO MINIMIZE HEAT TRANSFER FROM SAID ENCAPSULATION TO THE SIDE WALLS OF SAID CATHODE, THE TOP OF SAID CAPSULE BEING IN INTIMATE HEAT TRANSFERRING CONTACT WITH THE UNDERSIDE OF THE TOP WALL OF SAID CATHODE AND CONDUCTORS EXTENDING FROM SAID COIL EXTERNALLY OF SAID ENCAPSULATION AND FORMING LEADS FOR ENERGIZING SAID HEATING ELEMENT. 